1. Field of the Invention
This invention relates generally to a structure of a frame for a motorcycle and more particularly to an improved frame structure with increased rigidity in the area of the pivot plate.
2. DESCRIPTION OF BACKGROUND ART
A known frame structure for a motorcycle is disclosed in Japanese Utility Model Publication Application No. Heisei 2-2714. This frame structure is constructed such that a pair of left and right main pipes and a pair of left and right down pipes extend from a head pipe and cross pipes are provided between rear portions of the left and right main pipes and the left and right down pipes. A pair of vertical pipes are connected between the cross pipes. The main pipes, the down pipes and a rear frame are coupled into a unitary member by way of side stays (pivot plates). A pivot for supporting a front end portion of a rear fork thereon is provided on each of the side stays.
With the frame structure described above, sufficient rigidity can be exhibited against a load acting in a vertical direction, but when force acts in a leftward or rightward direction, a distortion of the frame likely occurs around the side stays (pivot plates). The tendency is high particularly in the case wherein the main pipe is extended to a further rear position in order to avoid interference with another member such as an exhaust pipe. In that case, the mounting positions of the pivot plates also extend to further rear positions and the tendency for distortion is increased.